This invention relates to a process for purifying certain poly(ether ketone ketones), sometimes abbreviated herein as PEKK's.
PEKK's form a class of thermoplastic polymers especially useful in applications where their high melting temperatures and excellent mechanical properties are important, e.g., in aircraft interior panels and in electrical insulation for electrical conductors.
PEKK's are usually made, either directly or via an oligomer intermediate, from an aromatic ether, most frequently diphenyl ether, and an aromatic diacid dichloride or mixture of two or more such diacid dichlorides, most frequently a mixture of terephthalyl chloride and isophthalyl chloride, in the presence of a Friedel-Crafts catalyst, usually aluminum chloride, AlCl.sub.3.
The catalyst or catalyst residue is difficult to remove from the PEKK because the metal, especially aluminum, is complexed by the polymer carbonyl and ether groups and thus held very tightly. Yet, if not removed, the catalyst residue will promote polymer degradation and discoloration, and therefore is an undesirable contaminant.
Maresca describes in U.S. Pat. No. 4,611,033 a process for stabilizing a poly(aryl ether ketone) such as PEKK, which comprises treating the polymer with a dicarbonyl chelating agent. The dicarbonyl chelating agents used in the reference process have the formula ##STR1## where R.sub.1 is an alkylene or cycloalkylene radical, and R.sub.2 is hydrogen, alkyl, cycloalkyl, or aryl. A chelating agent exemplified in the reference is 2,4-pentanedione. Even though 2,4-pentanedione (also known as acetylacetone) is commercially available, such compounds are quite expensive an economically attractive in certain applications, especially where the PEKK does not have a sufficiently high value in use. It, therefore, would be desirable to provide a less expensive but at least equally effective process for removing metal residues from PEKK's.